One existing type of optical device is an optical add/drop multiplexer (OADM), which has several collimating lenses and several filters, and which has an input fiber and several output fibers that are each physically coupled to a respective collimating lens. Optical radiation is supplied to the OADM through the input fiber and its associated lens, and impinges on the first of the filters, which reflects a portion of the radiation and passes a further portion of the radiation to a lens associated with one of the output fibers. The reflected portion of the radiation travels to another of the filters, which reflects a portion of that radiation and passes another portion of it to another lens associated with another output fiber. This sequence may continue for several more filters, lenses and output fibers.
In a known approach for manufacturing an optical device of this type, each output fiber is physically and optically coupled to its associated lens before the lenses are installed into a support arrangement which maintains them in an operational configuration. As a practical matter, the filters used in these devices are not ideal. For example, the filters may have surfaces on opposite sides thereof which are not parallel, and which thus produce a small amount of refraction in the radiation which passes therethrough. Due to this refraction, radiation which has passed through the filter travels away from it along a path which has a small angle with respect to the path that this radiation would have followed if the filter conformed to ideal characteristics.
In order to compensate for the refraction of this radiation, various techniques are used to permit independent adjustment of the lenses for each of the output fibers relative to the member which supports them, so that each such lens accurately focuses radiation on an end of the output fiber associated with it. In general, this approach involves the need for four degrees of freedom in the positioning of each such lens, including linear movement parallel to each of two translational axes that are normal to each other and to the optical axis of the collimating lens, and also pivotal movement about these two translational axes. These four degrees of freedom make accurate alignment relatively difficult, especially since collimating lenses are often more sensitive to rotational movement than to linear movement by a factor of approximately 3. Moreover, once alignment is obtained, it may be difficult to maintain it through environmental conditions such as vibration and temperature changes. Another consideration is the fact that this traditional approach involves a significant amount of manual labor and is thus expensive and time consuming in a production environment.